Update the minimum supported android version from 21 to 24

[reidbaker]

go/rfc-flutter-android-m-deprecation

• Bump the template build.gradle minSdkVersion number.
• Add auto-migration for existing projects with 21-23
• Update minSdkVersion in the engine AndroidManifest.xml (and tests)
• Update all code examples/tests to Android version 24
• Create a breaking change notice and migration guide on https://docs.flutter.dev/release/breaking-changes
• Update minimum supported Android version to 24 in flutter/website
• Update all existing Flutter maintained plugins to Android 24 (after a stable has min sdk 24)
• Audit native android code in any plugins that needed to be update to 24 as a min sdk version for pre 24 codepaths and delete those codepaths.
• Document the steps required to deprecate an android version in docs/platforms/android/
• Close all github issues against the deprecated platform
• Migrate all g3 apps internal tracking issue b/450579100

Past prs that may help:
flutter/engine#43822
flutter/packages#4459

[stuartmorgan-g]

Update all existing Flutter maintained plugins to Android 24

I would recommend a separate bullet point to track scrubbing all the native code and plugin READMEs for anything specific to pre-24 support. In the past we've sometimes done the mechanical steps of changing the min deployment version but forgotten the step of cleaning up unnecessary code.

[reidbaker]

Update all existing Flutter maintained plugins to Android 24

I would recommend a separate bullet point to track scrubbing all the native code and plugin READMEs for anything specific to pre-24 support. In the past we've sometimes done the mechanical steps of changing the min deployment version but forgotten the step of cleaning up unnecessary code.

done

[reidbaker]

@stuartmorgan-g I forget the preferred order. Would you like me to update the engine/framework then update the plugins once there is a new version published to stable or would you like me to update the plugins to have a higher min first.

[stuartmorgan-g]

Would you like me to update the engine/framework then update the plugins once there is a new version published to stable

This one; the other versions can break people's compilation because we don't have a way to feed OS version requirements into the resolver. By changing Flutter first to require 24 first, and then updating the plugin's min Flutter SDK version to that Flutter version, we can use existing resolver information (the Flutter constraint) to prevent anyone building for 21-23 from getting a version that requires 24 and thus breaking their build.